I'm looking into rebuilding my 500 EFI, it has about 450 hours on it but still runs like a champ. It has already had new valves, springs, etc.. put in around 200 hours. When I rebuild it and punch it out to a 509, do I need to replace the heads or just have them machined and replace the springs and valves?

Really all depends on your budget. If you don't want to spring for New heads I'd just have them gone through again. If you want to spend the extra and get a little more power then sell your old heads and put it towards the new ones.

If you are keeping it stock, then just have the machine shop do a valve job, and replace the springs, retainers/locks and guides.

If its within the budget, replace the exhuast valves. However, if there is any signs of water, you have to replace the exhuast valves. I agree.... if going for a stock rebuild, no need to replace the heads. It may cause more problems than it would be worth.

$2500 will get you a beauty set of AFRs.

Well frankly, I am a boater on a budget. I have a 1 and 3 year old so I'm not trying to build an engine that's going to set the world on fire speed wise, my #1 goal is reliability (turn key). It's starting to get high in hours so that's why I was thinking about doing a good rebuild so I wouldn't have anything to worry about for another 500 hours from now.

I would just have the heads redone then. Also. .If your cylinder walls are within spec I would just hone instead of boring it to a 509

One of the guys I was talking to mentioned something about a valve seat falling out if they weren't replaced or something along those lines, does that make sense?

Just take them to a reputable machine shop, they will tell you exactly what needs to be done.

If the heads have been run in salt water for 10+ years it would be a good idea to replace the castings,the salt water eats them up from the inside and can cause the exhaust seats to fall out or get a pinhole,if the budget allows new castings are the way to go,can also get a few bucks for your old ones if still good